This invention relates generally to micro-electromechanical structures (MEMS) processes, and more specifically to electrical connections from the inside to the outside of a sealed cavity formed in MEMS processes.
One method of producing micro-electromechanical structures (MEMS) is by bonding a patterned silicon wafer to a glass (usually pyrex) substrate. Portions of the silicon wafer are etched away, leaving a mechanical silicon structure anchored to the glass substrate. The process is initiated with a glass wafer. A cavity is formed in the wafer using a wet or dry etching process. A depth of the etch determines a separation between the structure's capacitive elements. Metal layers are deposited and patterned on the glass, forming conductive electrodes and interconnects. A heavily boron doped (p++) epitaxial layer is grown on a separate, lightly doped silicon substrate. A pattern is etched into the silicon wafer to a depth greater than the thickness of the epitaxial layer. The glass and silicon wafers are bonded together using anodic bonding. Using an etchant that etches lightly doped silicon but not p++ silicon, the undoped portion of the silicon substrate is etched away, leaving the freestanding microstructures. Such a process is generally referred to herein as a silicon-glass MEMS process.
The mechanical structures are exposed to the ambient environment during operation. Therefore, if the mechanism requires a special operating environment, or simply protection from the ambient environment, the protection must be done at a packaging step.
One packaging step is to form the mechanical structures in the silicon-glass MEMS process within hermetically sealed cavities. One known method for forming such cavities is to bond a silicon structure, containing a recess, to the glass wafer. The recessed portion, which does not extend all the way through the p++ silicon layer, forms the cavity after bonding. Unfortunately, using this method it is difficult to make a silicon structure, for example, a vibrating sensor, that is completely enclosed by the cavity but not connected to the cavity walls.
Another method is to bond a second glass wafer, containing recesses, on top of the previously fabricated glass/Si wafer. At least part of the silicon structure is a continuous seal ring that completely surrounds, but is not connected to, a second silicon structure (i.e. the vibrating sensor). The second glass wafer is bonded to the seal ring, but not to the second silicon structure, forming the cavity.
However, it is desirable to have electrical leads extending from outside the cavities into the hermetically sealed cavities, without breaking the seal. A seal is much more difficult to attain since formation of the electrical leads form uneven topography on the glass substrate surface. Small gaps occur where the leads are formed on the substrate. The gaps result in an uneven sealing surface and result in unbonded areas which break the hermetic seal. Anodic bonding is one method that has been used in an attempt to alleviate this problem. However, anodic bonding can only be accomplished over uneven areas that fluctuate by about 200 angstroms or less, and a long term leak rate for anodic bonding seals is unknown. Therefore, in order to have leads that extend through a reliable hermetic seal, a topography used in making the seal should be substantially flat.